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Mount and Mirror (MM)
Working Group Report
Presented by Razmick Mirzoyan
On behalf of the MM working group
Max-Planck-Institute for Physics, Munich
March 2, 2007
R. Mirzoyan: CTA Paris Meeting; Mount
& Mirror WG Report
Sizes of few well-known telescopes
DISH
Ø 5m = small
FoV
4° - 5°
normal
Ø 10m = medium
Ø 20m = large
6° - 12°
wide
MAGIC – 239m²
HESS I – 108m²
HEGRA – 8.3m²
March 2, 2007
R. Mirzoyan: CTA Paris Meeting; Mount
& Mirror WG Report
Ø 30m = huge
HESS II – 600m²
Size of a telescope
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
Currently we have quite some experience with
constructing telescopes of sizes ranging from
(3 – 17) m in .
Within next 2 years we will start collecting
experience also with a  28m H.E.S.S.
telescope.
March 2, 2007
R. Mirzoyan: CTA Paris Meeting; Mount
& Mirror WG Report
Choice of the size of a telescope

The choice of the size of a telescope is dictated by the
desired
a) energy threshold
b) field of view (FoV)
c) optical resolution in the given FoV
d) location altitude
e) also, type of the used light sensors (future trend)
f) last but not least : available financial support
March 2, 2007
R. Mirzoyan: CTA Paris Meeting; Mount
& Mirror WG Report
The estimated energy thresholds
The Ethreshold shall scale with the  of a telescope as E ~ 1/{}²
Telescope
HEGRA
HEGRA CT-1
# telescopes x
mirror ,m
Trigger threshold, Analysis threshold,
GeV
GeV
5 x 4m
500
(500) 1000
1 x 4.3m
500
(750) 1000
CAT
1 x 5m
H.E.S.S.-I
4 x 12m
100
160
VERITAS
4 x 12m
~like H.E.S.S.-I
~like H.E.S.S.-I
3 (4) x 10m
250
500
MAGIC-I
1 x 17m
50
100
H.E.S.S.-II
1 x 28m
25
50
MAGIC-II
2 x 17m
30
50
CANGAROO-III
March 2, 2007
300
R. Mirzoyan: CTA Paris Meeting; Mount
& Mirror WG Report
IACTs comparative table
Telescope
HEGRA
H.E.S.S.
MAGIC
VERITAS
CANGA
ROO
#,
 (m)
5 +1
4.3
4 (+1)
12 (28)
1 (+1)
17 (17)
4
12
4
10
Mirror Area,
m²
(5 x 8.3) +
(1 x 10)
(4 x 108) +
(1 x 600)
(1 x 239) +
(1 x 239)
4 x 110
4 x 57
Reflector
Design
DaviesCotton
DaviesCotton +
(Parabolic)
Parabolic +
Parabolic
DaviesCotton
Parabolic
f/#
1.2
1.2
1.2
1
1
0.8
Weight, T
(5 x 2.4) +
1x4
(4 x 60) +
1 x 500
64 +
64
Geom. FoV
5 x 4.5° +
1 x 3.7°
4 x 4.5°
1 x 3°
3.9° +
3.9°
4 x 3.5°
4 x 3.2°
March 2, 2007
R. Mirzoyan: CTA Paris Meeting; Mount
& Mirror WG Report
Comparison example
In optical astronomy the cost of a telescope scales ~ {}2.6
It seems that the cost of an IACT can be scaled ~ {}2.2
Telescope
HEGRA
K€
H.E.S.S.-I
K€
H.E.S.S.-II
K€
MAGIC-I
K€
MAGIC-II
K€
4 x 60.-
450.-
250.-
260.-
4 x 15.-
120.-
39.-
44.-
Material +
5 x 70.-K€ 4 x 300.Engineering
4 x 50.-
3200.1000.-
720.-
820.-
Total Invest
Cost, K€
12000.-
4000.-
4600.-
Foundation
Rail system
March 2, 2007
no
<1000
12000.-
R. Mirzoyan: CTA Paris Meeting; Mount
& Mirror WG Report
Mechanical constraints
Space frame, dimensions, weight,… materials:
• Steel: very big experience, thermal expansion O’K, stiff, heavy
• Aluminium: innovative, expensive, twice higher thermal
expansion than steel, light. No corrosion.
• Carbon Fibre reinforced plastics: innovative, thermal expansion is
just 7 % from that of Al, very stiff, very light. Avoid excessive
heat. ~ 2 times more expensive than steel.
Thermal
expansion
Concrete
Glass,
ordinary
Steel
Al
Carbon fibre
reinforced plastic
(x10-5 °C-1)
1.2
0.9
1.3
2.4
0.2
March 2, 2007
R. Mirzoyan: CTA Paris Meeting; Mount
& Mirror WG Report
Exapmle CFRP frame: MAGIC-I
Deformation of the mirror dish
Camera support, bending vs. elevation
Possible hysteresis of deformations
March 2, 2007
R. Mirzoyan: CTA Paris Meeting; Mount
& Mirror WG Report
CTA Design criteria








Stereoscopic telescope system (>> 4 telescopes)
Alt-Az mount on rails
Davies-Cotton for small dishes, parabola for large dishes
(> 10m)
Segmented mirror facetts, possibly ~ 1m size for large dishes
F/D > 1
Robust, simple, reliable system (lifetime > 10 years), robotic
operation desired, keeping maintenance/operation effort low
Survival winds > 160 km/h
Consider protection during ice, snow, below 0 temperatures
March 2, 2007
R. Mirzoyan: CTA Paris Meeting; Mount
& Mirror WG Report
Design criteria




Weight of the camera (will depend on the level of integration of
electronics inside the camera)
Positioning and pointing accuracy
Stiffness of the structure and/or AMC system
Safety concept
 Human safety
 Protection of the instrument
 Redundancy in power, drives
March 2, 2007
R. Mirzoyan: CTA Paris Meeting; Mount
& Mirror WG Report
Mechanics



Tracking precision: like optical telescopes
Positioning accuracy, speed
Fast slewing: (MAGIC:180°/50sec; H.E.S.S.: ~twice slower;
VERITAS: 1°/1sec)
 Drive type
 Gear
 Tooth rims
 Friction
March 2, 2007
R. Mirzoyan: CTA Paris Meeting; Mount
& Mirror WG Report
Arrival Time Front


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No arrival time distortion for
the parabolic design
Special 2 component optical
telescope designs may
provide no (or almost no)
arrival time distortions
The wide-spread DaviesCotton design provides ~ 3-4
ns time smearing for existing
telescopes
March 2, 2007
The RMS of the pulses from g, hadrons
and µ after PMT and jitter simulations
Mirzoyan, Sobczynska, et al., 2006
R. Mirzoyan: CTA Paris Meeting; Mount
& Mirror WG Report
Light sensor type



Classical PMTs: currently used in all IACTs: Best
Photon Detection Efficiency
(PDE): ~ 30 %
HPDs with Ga AsP can offer interesting
alternatives. PDE ~ 45 % (QE: 50-55 %) (rather
expensive)
SiPM: mature product in 3-5 years time. Can
offer PDE ~ 60 %
March 2, 2007
R. Mirzoyan: CTA Paris Meeting; Mount
& Mirror WG Report
Field of View
Observations of the Crab Nebula with
H.E.S.S.; astro-ph/0607333v1
• Currently H.E.S.S. has the largest
FoV of all existing IACTs.
• Still, as one can see from the left
figure, the effective field of view is
„only“ ~3° in  (@ 3db=0.7 level)
• One may think either to provide a)
a „large enough“ FoV, so that it can
include inside the largest possible
gamma source ( 5°  ?) or b) really
large FoV of (10-20)°  for the all
sky survay.
March 2, 2007
R. Mirzoyan: CTA Paris Meeting; Mount
& Mirror WG Report
Prime focus optics: what is feasible ?
Schliesser & Mirzoyan, Astrop. Phys., 2005
3´ optical resolution
March 2, 2007
R. Mirzoyan: CTA Paris Meeting; Mount
& Mirror WG Report
Wide FoV
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Also, the (10-20)° FoV can offer the advantage of very high
optical angular resolution of ~ 1 arc minute everywhere in the
FoV.
Simulation results indicate further improvement of the angular
resolution, especially at low energies, when moving from
currently used
(3-6)´ towards 1´.
A proper design can offer almost no distortion of the arrival
time front.
Schmidt design: can offer interesting solutions for Wide FoV
Interesting recent design, Ritchey-Chretien: Vassiliev, et al.,
astro/ph 0612718
March 2, 2007
R. Mirzoyan: CTA Paris Meeting; Mount
& Mirror WG Report
Wide FoV
March 2, 2007
R. Mirzoyan: CTA Paris Meeting; Mount
& Mirror WG Report
Which mount is better ?

1.
2.

The above question is very much justified:
A heavy and stable mount, with marginal
deformations: approach chosen by HEGRA,
H.E.S.S., VERITAS
A light-weight mount that allows for small
deformations, which are actively correctd for:
approach chosen by MAGIC.
In the 2nd case the telescope consists of (mount +
active mirror correction (AMC) system).
March 2, 2007
R. Mirzoyan: CTA Paris Meeting; Mount
& Mirror WG Report
Example Active Mirror Control
(AMC) System
Star adjust option of AMC:
On figure left one can see
~12 % of all mirrors in the
Camera centre, the rest are
Defocused.
All the mirror can be
Checked in 8 such figures.
This will take ~ 10 minutes
time + some off-line work.
In addition, the reflectivity
of all individual mirrors can
regularly be measured.
March 2, 2007
R. Mirzoyan: CTA Paris Meeting; Mount
& Mirror WG Report
Which mount is the best ?


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We have collected positive experience with AltAz H.E.S.S. and MAGIC type mounts on rails.
A combination of a stiff mount + an AMC
system could offer an interesting solution.
This can be interesting for large and huge
telescopes, especially for large zenith angle
observations.
We have enough expertise to cope with this
problem.
March 2, 2007
R. Mirzoyan: CTA Paris Meeting; Mount
& Mirror WG Report
H.E.S.S.-2 mirrors
H.E.S.S.-I
mirror
March 2, 2007
R. Mirzoyan: CTA Paris Meeting; Mount
& Mirror WG Report
H.E.S.S.-2 mirror candidates
March 2, 2007
R. Mirzoyan: CTA Paris Meeting; Mount
& Mirror WG Report
H.E.S.S.-2 mirrors
March 2, 2007
R. Mirzoyan: CTA Paris Meeting; Mount
& Mirror WG Report
All-Al MAGIC-I mirrors
The MAGIC-I mirrors have size 0.5 m x 0.5 m. 4 of them are set together, forming a panel 1 m x 1 m.
MAGIC-I uses 956 mirrors of size 0.5 m x 0.5 m. They are grouped in 239 panels of 1 m x 1 m.
The single 0.5 m x 0.5 m mirrors are obtained by gluing two aluminum sheets on a “honeycomb”
structure. After rough pre-milling, a diamond “fly-cutting” of the surface will generate a spherical
surface of a desired radius, having a microroughness of about 100 Å
March 2, 2007
R. Mirzoyan: CTA Paris Meeting; Mount
& Mirror WG Report
Upgrade of manufacturing process
of MAGIC mirrors made in Padua
Monolithic 1 m x 1 m mirrors do not need a support panel and the pre-alignment.
A sandwich formed by two aluminum skins with inserted an “honeycomb” structure is assembled.
The aluminum skins are pre-curved and glued on the honeycomb using a concave spherical mould.
Afterwards a diamond milling of the surface is performed to refine the spherical surface. Very
positive test results!
March 2, 2007
R. Mirzoyan: CTA Paris Meeting; Mount
& Mirror WG Report
Cold Slumping Approach
New INAF-OAB technology in collaboration with Padua and with
Medialario Techn. – foresee the cold shaping of a sandwich made with 2
thin glass sheets (1.7mm) glued on an Al honeycomb (20mm) structure.
While the radius of curvature of the mirror is large and the thickness of
the glass sheet is rather thin, it can be conformed to the shape of a
convex master by means of vacuum suction.
Advantages of the glass-faced panels:
 Low cost. Lightweight. Doesn’t need polishing either of the glass/master.
 These characteristics permit to have:
 Competitive costs compared to other technologies
 Superior performance
 High production speed
March 2, 2007
R. Mirzoyan: CTA Paris Meeting; Mount
& Mirror WG Report
Mould and panel produced in Medialario Tech.
Aluminum master 1040 x 1040 mm
Front and rear of a produced segment
Size = 985 x 985 mm Weight = 10 Kg.
Nominal radius= 35 m
March 2, 2007
1m x 1m mirror tests at ETH
R. Mirzoyan: CTA Paris Meeting; Mount
& Mirror WG Report
Point Spread Function
90% of the light inside 0.9 mrad
80% of the light inside 0.6 mrad
Achieved parameters already within the MAGIC-II specifications
March 2, 2007
R. Mirzoyan: CTA Paris Meeting; Mount
& Mirror WG Report
Developments with potential
• Dielectric mirror coatings can offer an interesting alternative:
one can „boost“ the mirror reflectivity in the entire wavelength
range 300-700 nm to ~ 100 %. Needs development work.
• Aspheric mirrors are another interesting development direction.
They can offer the advantage of better PSF.
March 2, 2007
R. Mirzoyan: CTA Paris Meeting; Mount
& Mirror WG Report
Conclusions MM


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Telescope Mount: quite some experience with several
types, that can be used in CTA.
Mirrors: experience with several types, the cost varies
between ~(1.2 – 2.5).-k€/m² for large quantities. We
will need to use a robust technology, the reflectivity
shall not seriously deteriorate within 5-10 years.
Wide FoV telescope designs may offer interesting
solutions, we need to explore their potential.
March 2, 2007
R. Mirzoyan: CTA Paris Meeting; Mount
& Mirror WG Report